This invention relates to the separation of nitrogen from a natural gas containing nitrogen over a wide concentration range to form nitrogen and natural gas product streams under elevated pressure without incorporating means for recompression of the separated products.
Petroleum production methods currently are utilizing high pressure nitrogen injection to maintain well head pressure for enhanced oil and gas recovery. As nitrogen is injected, the natural gas from the well containing methane and associated hydrocarbon liquids also contains nitrogen which increases in amounts over the life of the nitrogen injection project. For this reason, the natural gas containing nitrogen must be separated to reject the nitrogen and to form purified natural gas feedstocks suitable for utilization as fuel or chemical feedstocks.
U.S. Pat. No. 3,797,261 discloses the separation of natural gas containing nitrogen into a low-nitrogen fraction and a high-nitrogen fraction by distillation in a single distillation column by expanding the high nitrogen fraction with the performance of work and using the resulting refrigeration to condense vapor in the upper section of the column while additional reflux is provided by vaporizing a recycle medium in heat exchange relation with vapor in the column. The high nitrogen mixture, having been expanded, is exhausted at atmospheric pressure.
Linde Reports On Science And Technology 15/1970, pp. 51-52, shows a process for separating nitrogen from natural gas containing a fixed nitrogen content, i.e., 15% nitrogen. A methane cycle, operating on the principal of the heat pump, is utilized in the process to provide the refrigeration. The overhead nitrogen fraction from the distillation column is depicted as a supplemental means to subcool the methane prior to methane expansion to provide refrigeration to the column.
In a nitrogen injection process to maintain well head pressure, the extracted gas increases in nitrogen content such that natural gas from the well can contain nitrogen over a wide range of concentration, e.g., generally from 5 to 85%. Conventional processes are limited in ability and may be ineffective for separating nitrogen from natural gas over such a wide range of nitrogen content to produce nitrogen and natural gas product streams under elevated pressure. Further, conventional processes for separating nitrogen from natural gas containing nitrogen and having a significant carbon dioxide content are restricted by carbon dioxide freezing or solidifying in the process equipment.